On internal combustion engines having one or more pistons that displaces inside respective cylinders, each piston comprising one or more rings, which are subjected to severe strains when the engine is running.
One of the ways to guarantee resistance to wear of a ring, so that it can have long useful life, is the application of one or more coating layers onto the base metal from which it is built. Thus, the coating, developed specifically for resisting wear and abrasion, can maintain the performances properties of the ring, even after millions of cycles of piston displacement inside the cylinder.
One of the most common surface treatments between piston rings is achieved by means of a nitriding layer on the wing surface, but other coatings may be deposited onto the ring surface.
It should be noted that the nitrided layer comprises two layers, which are the diffusion layer and subsequently the white layer composed by iron nitride phase.
In the case of rings of the prior art, this is not different, and therefore the traditional processes of making nitrided piston rings have both the diffusion layer and the compound layer. As a characteristic, the while layer is very hard, fragile and porous. In turn, the live edges of a piston ring are tension concentration points. When these regions receive the nitrided white layer, there is a significant increase in the concentration of tensions, which leads to a more rapid fracture of the ring.
FIG. 1 shows exactly the above-described phenomenon. An attentive observation of FIG. 1 shows how the crack of the component began at the left lower chamfer, extending into the total fracture, a typical situation with piston rings.
Although there are evidences showing that a compressive residual tension is advantageous on preventing the formation of cracks, the same compressive residual tension is different when it exists at the edges of a piston ring.
As shown in FIG. 2, a nitrided layer on the ring surface in a region close to an edge results on vectors V1 and V2. The combination of the compressive tensions of vectors V1 and V2 in the edge region result in a third vector V3, which contributes to forming the crack and may lead to fracture of the component when it is subjected to severe functioning conditions.
It should be noted that the piston rings further receive hard films, generally generated by physical vapor deposition (PVD) or chemical vapor deposition (CVD). In the solutions of the prior art, the ring surfaces that will receive the hard film require removal of the hard layer from the compound, with a view to improve adhesion of such a hard film.
In general, the methods used for removing the white layer require machining the ring by a grinding process. These processes are known as potential generators of cracks due to the cutting forces generated during the machining. Considering that the nitrided layer in the chamfer regions already have high residual tension, said machining may cause little fractures on these surfaces. Given the cyclic physical strains to which the piston ring is subjected, a small fracture may easily lead to break of the component.
The results presented in the table below show the increase in residual tension after machining the nitrided layer of the prior art. It should be noted that the residual tension after nitriding increased by about 300 Mpa to about 600 Mpa after the machining.
TABLE 1a comparison between the values of residualtension after nitriding and after machining.After nitridingAfter machiningMPaMPaSample180°180°1st ring−229−5342nd ring−198−7453rd ring−312−6194th ring−296−6175th ring−226−728
In order to improve the fracture resistance of nitrided piston rings, which naturally may also receive a hard film, various alternatives have been developed. One first prior technique representative of such attempts is presented by U.S. Pat. No. 6,698,736, which refers to a piston ring on which the nitrided layer is completely removed from the chamfer regions, since its presence is harmful to the fracture resistance.
Thus, the document discloses that it is necessary to carry out nitriding with the ring having its live edges, or with extremely reduced chamfers, because otherwise it is not possible to remove completely the nitrided layer in such regions. As explained above, these regions are strong concentrations of tensions that increase with nitriding. In this regard, the technology proposed by U.S. Pat. No. 6,698,736 is vulnerable, because, if the nitrided layer is not completely removed from the edges, the fact that the residual nitrided layer contains residual tensions will impair the fracture resistance of the component.
Another prior technique is presented by the Japanese patent application JP 2002-061746, which describes a piston ring having a metallic base of cast iron or steel. Such a ring comprises a concave portion in the region of the axial center of the outer face, where a nitrided layer is formed. After removal of the nitrided layer from the chamfer and corner regions, a hard ceramic film layer is deposited onto the outer face. Just as happens in the technology presented by the above patent, the nitrided layer is removed from the corners by machining or by a grinding process to form the chamfers. In order for this technology to be feasible, it is necessary that the corners should have live edges before receiving the nitrided layer, otherwise it will not be possible to remove the nitrided layer totally. Thus, the machining of the nitrided live edges that have a high level of tensions lead to the generation of cracks that will impair the resistance of the ring, causing fracture thereof.
Japanese document JP3090520 discloses another solution found in the prior art to the problems described above. In this case, a piston ring of stainless steel is provided with nitrided layers on the upper, lower surfaces and on its inner diameter, receiving also a ceramic film by ionic deposition onto the peripheral outer surface. It should be noted that, in this case, the nitriding treatment takes place after the PVD, so that the PVD hard film will act as a barrier that prevents nitriding. The disadvantage of this invention is due to the fact that only the PVD film will support the slide surface that undergoes wear. It should be noted that, if the PVD film is totally consumed, there will be no nitriding layer under the PVD film capable of continuing to support the wear. Thus, the life of the piston ring depends completely on the durability of the PVD film and, is the latter becomes extinguished, the soft material that composes the metallic base will be exposed to the cylinder wall, which causes wear of the ring and, as a result, the break of the motor.
Thus, the solutions presented by the prior art have two great drawbacks, which have not been eliminated so far. On the one side, in the technologies where nitriding is carried out on the whole ring, one needs machining processes capable of removing it totally, so that the deposition of the hard ceramic film can have good adhesion. On the other hand, the fact that the hard ceramic film is deposited directly onto the metallic base of the piston ring brings the drawback of the life of the piston ring depending completely on the durability of the PVD film.
Therefore, no solution has been found to enable nitriding over the whole surface of the piston ring and enable good adhesion of a hard ceramic film without the need for any intermediate machining process, that is, deposited over the nitrided layer.
The present invention has the objective of providing a piston ring designed for use on internal combustion engines or compressors, provided with a special nitriding surface treatment, so that at least one of its nitrided surfaces will receive a hard ceramic film with good adhesion and without the need for machining processes.
The present invention has also the objective of providing a piston ring provided with a nitriding surface treatment applied onto the ring, the chamfers of which are in their final geometry, and that is capable of minimizing the internal tensions of the nitrided layer.
The present invention has a further objective of providing a piston ring that will guarantee an extended life of the piston ring, so that, when the PVD film has been consumed, a nitrided layer existing under the PVD film will enable the functioning of an engine in full conditions.